Nonaqueous redox flow batteries (RFBs) hold the potential for high energy density grid scale storage. See R. M. Darling et al., Energy Environ. Sci. 7, 3459 (2014); and B. R. Chalamala et al., Proc. IEEE 102, 976 (2014). While aqueous chemistries are limited to the 1.5 V potential window of water, many nonaqueous electrolytes with stability ranges greater than 4 V exist, allowing for increased cell voltages and corresponding energy densities. See W. Wang et al., Adv. Funct. Mater. 23, 970 (2013); and A. Z. Weber et al., J. Appl. Electrochem. 41, 1137 (2011). To best take advantage of the wider potential window of nonaqueous electrolytes, many groups have created highly reversible, electrochemically novel molecules for non-aqueous catholyte and anolyte chemistries ranging from complex, fully organic redox molecules with light weight high current and high efficiencies, to redox-active organic ligands complexed to metal ions, to redox activity centered in the cation core, or to iodide anions. See J. Huang et al., Sci. Rep. 6, 32102 (2016); J. D. Milshtein et al., Energy Environ. Sci. 9, 3531 (2016); C. S. Sevov et al., J. Amer. Chem. Soc. 138, 15378 (2016); C. S. Sevov et al., J. Amer. Chem. Soc. 127, 14465 (2015); C. S. Sevov et al., Adv. Energ. Mater., 7, 1602027 (2017); A. P. Kaur et al., Energy Technol. 3, 476 (2015); L. Su et al., J. Electrochem. Soc. 161, A1905 (2014); W. Wang et al., Chem. Commun. 48, 6669 (2012); R. A. Zarkesh et al., Dalton Trans. 45, 9962 (2016); J. Mun et al., J. Electrochem. Soc. 15, A80 (2012); N. S. Hudak et al., J. Electrochem. Soc. 162, A2188 (2015); S. Schaltin et al., Chem. Commun. 52, 414 (2016); J. Suttil et al., J. Mater. Chem. A 3, 7929 (2015); M. Miller et al., J. Electrochem. Soc., 163, A578 (2016); L. J. Small et al., J. Electrochem. Soc. 163, A5106 (2016); L. Cosimbescu et al., Sci. Rep. 5, 14117 (2015); H.-S. Kim et al., J. Power Sources 283, 300 (2015); C. Jia et al., Sci. Adv. 1, e1500886 (2015); and H. Chen and Y.-C. Lu, Adv. Energ. Mater., 1502183 (2016).
These chemistries often possess large cell voltages and stable redox activity, though are limited by the solubility of the redox-active species, with only a few exceeding 1 M. This limited solubility severely hinders the widespread deployment of RFBs. At a RFB energy density of 50 Wh/L, 230 times more volume is required to house a fully charged RFB electrolyte than the same energy content of natural gas. The fundamental difference between RFB electrolytes and natural gas lies at the molecular level; every molecule of natural gas participates in the energy-generating reaction, compared to <5% for many RFB electrolytes. Therefore, a need remains for a method to increase the energy density in RFB electrolytes.